Lumber is a product of nature having a variety of grain structures and discontinuities which significantly affect the strength of each particular lumber piece. In order to classify lumber according to the varying strength of each piece the lumber industry has developed grading criteria which are helpful at predicting strength ranges into which a piece of lumber should be sorted. Grading of lumber has typically been accomplished visually by noting the frequency, placement and size of discontinuities such as knots and separations. Such visual grading techniques are not sufficient to provide an accurate indication of strength for any particular piece. They do provide a general average strength value and range of strength values which can be used in engineering.
Grading of lumber has been more recently improved using a technique called mechanical stress rating of lumber. Such technique applies a flexural load to a piece of lumber. The flexural properties of the lumber piece are then analyzed to predict strength and appropriately grade the piece of lumber. Unfortunately, the accuracy of determining strength using mechanical stress rating techniques is still limited since it attempts to predict lumber strength from non-strength information, namely, flexure. Lumber graded in this manner still shows variability in strength which must necessarily be accommodated in determining allowable design limits.
The current invention is designed to apply a test or proof load which the lumber piece must withstand. This technique increases the design assurance over prior art grading techniques by providing lumber which is known to have passed a given applied load.